Code in module _upload.py, line 453 of method _process_initiate_response reads:

        _helpers.require_status_code(
            response,
            (http_client.OK,),
            self._get_status_code,
            callback=self._make_invalid,
        )

However, initiating an upload against https://storage.googleapis.com yields a 201 Created response, so it fails to comply.

Fixing this is as simple as adding http_client.CREATED to line 455:

(http_client.OK, http_client.CREATED),